The present invention relates to a track search control circuit for controlling track search of an optical disc and an optical disc drive equipped with a track search control device with the use of the track search control circuit, which are used, for example, for optical disc reproducing apparatuses such as a CD (Compact Disc) ROM drive for a computer system, a DVD (Digital Versatile Disc) drive, etc. and for optical disc recording/reproducing apparatuses for a CD-R, a CD-RW, a DVD-RAM, etc.
Conventionally, in optical disc systems for the DVD, the CD, etc., when the track search of the optical disc is performed, track traversing direction/frequency of a laser beam spot are detected using amplitude information of a tracking error signal and a readout signal at a time when the laser beam spot traverses the track, and the velocity control of the track search is performed.
That is, when the track search is performed, the track search control circuit is given the track search direction BWFW and the necessary number of tracks to be searched by a system controller, and generates track traversing information of the laser beam spot from a tracking error signal TE and a readout ripple signal RFRP. Then, acceleration/deceleration energy necessary for the track search is calculated from the track traversing information, the track search direction, and the number of tracks to be searched, and a signal indicative of the energy is added to an input of a tracking servo equalizer.
Now, along with the increase in a disc angular velocity in reproducing the optical disc in connection with the competition for reproduction speed of optical disc systems, the necessity of performing the track search under a condition of a large angular velocity of the disc has been arisen. However, in the case where the disc angular velocity is large, the variation of eccentric acceleration of the disc is large, and hence it is likely to be difficult to control the track search velocity effectively in the conventional track search method.
Hereafter, regarding this respect, a configuration of a conventional velocity error detection circuit for the track search control circuit will be described with reference to FIG. 11.
In the velocity error detection circuit of FIG. 11, a velocity error detection counter (FVCTR) 81 measures a semi-track interval. The velocity error detection counter FVCTR 81 is a 5-bit counter, whose count is incremented by a target velocity clock FVCK. Therefore, the target velocity clock counter (FVCTR) 81 can count thirty-two clocks (FVCKS) and when the counter reaches the full-count, a full count detection signal FULL-DET becomes logic “H”. Further, when the velocity error detection counter (FVCTR) 81 counts the full count, a gate 83 for the target velocity clock FVCK is turned off using an inverted signal of the full count detection signal FULL-DET by the inverter circuit 82 in order that after the full count the full count value does not return to zero by the next target velocity clock FVCK.
On the other hand, a normal direction on-track/off-track signal FVCLR is a pulse generated at the normal direction on-tracking/off-tracking time, which clears the FVCTR 81. It should be noted that the “normal direction” means a case where a direction in which the lens of the pick-up mechanism is moved with regard to the track is the same as a track search direction given by a system controller.
A data latch signal FVLP is a pulse generated just before the generation of the normal direction on-track/off-track signal FVCLR. The data latch signal FVLP latches (the count value of the FVCTR−15)×(+1) or (the count value of the FVCTR−15)×(−1) in the TKIC (track search velocity control) register 85. Specifically, when the track search is to be performed in the outward direction, the BWFW signal from the system controller is goes to a logical low level, and in this time, (the count value of the FVCTR−15)×(+1) is latched by the TKIC register 85. On the other hand, when the track search is to be performed in the inward direction, the BWFW signal from the system controller goes to a logical high level, and in this time, (the count value of the FVCTR−15)×(−1) is latched by the TKIC register 85.
data including the count value (velocity measurement data) of the FVCTR 81 and the track search direction BWFW given by the system controller into a TKIC (track search velocity control) register 85. These normal direction on-track/off-track signal FVCLR and data latch signal FVLP are generated using the tracking error signal TE and the readout ripple signal RFRP.
Therefore, the velocity measurement result by the FVCTR 81 that counts up the target velocity clock FVCK is loaded into the TKIC register 85 at the data latch signal FVLP generated just before the generation of the track pulse, and is cleared by an inverted signal of the normal direction on-track/off-track signal FVCLR by an inverter circuit 84. In this case, a velocity measuring period of the FVCTR 81 is a half-track period, and acceleration/deceleration data stored in the TKIC register 85 is outputted in a half-track period next to the velocity measuring half-track period.
FIG. 12 is a characteristic diagram showing the relation between the track search velocity error and the number of the FVCK counts of the velocity error detection counter (FVCTR) 81 of FIG. 11.
FIG. 13 is a characteristic diagram showing the relationship between the FVCTR value detected as the velocity error and the value stored in the TKIC register 85 in the outward direction search, i.e., BWFW=“L”.
FIG. 14 is a characteristic diagram showing the relationship between the FVCTR value detected as the velocity error and the value stored in the TKIC register 85 in the inward direction search, i.e., BWFW=“H”.
FIG. 15 is a waveform chart showing output timing for a velocity error detection result in the velocity error detection circuit of FIG. 11 in a case where a track search in the outward direction is taken as an example.
In FIGS. 11 and 15, the target velocity clock FVCK is a clock having a frequency sixteen times that of the actual target velocity (half-track traversing target frequency). Accordingly, if the FVCTR 81 counted sixteen clocks until the velocity measurement result of the FVCTR 81 is latched in the TKIC register 85 at the data latch signal FVLP, it is the case where the actual velocity is equal to the target velocity (i.e. the velocity error being zero), the value zero (“0”) is stored in the TKIC register 85.
Moreover, if the FVCTR 81 counted less than sixteen clocks until the velocity measurement result of the FVCTR 81 is latched in the TKIC register 85 at the data latch signal FVLP, it is the case where the actual velocity is higher than the target velocity, and a negative value in the outward direction search or a positive value in the inward direction search is stored in the TKIC register 85.
On the contrary, if the FVCTR 81 counted more than sixteen clocks until the velocity measurement result of the FVCTR 81 is latched in the TKIC register 85 at the data latch signal FVLP, it is the case where the actual velocity lower than the target velocity, and a positive value in the outward direction search or a negative value in the inward direction search is stored in the TKIC register 85.
In this way, error data between the target velocity and the actual track traversing velocity is modified by the track search direction signal BWFW and stored in the TKIC register 85, which becomes an input of addition of the tracking servo equalizer to effect the addition of a signal indicative of acceleration/deceleration energy in a tracking direction.
The velocity control method for the track search mentioned above referring to FIGS. 11 to 15 has an advantage that the track search velocity can be controlled accurately based on the measured velocity error (in multiple values). However, as will be mentioned below, it is difficult to perform the track search in the optical disc system for the DVD, the CD, etc. stably when the disc is reproduced at a multi-speed, because the timing of outputting the velocity measurement result stored in the TKIC register 85 is always in a next half-track period next to the period (half-track) when the velocity measurement was performed and because there may exist the eccentricity of the axis of rotation of the optical disc.
That is, generally in the case of a removable disc, when the disc is manufactured, the center of the disc made in a donut-shape is not necessarily in the center of a track formed in a spiral manner from which the signal is read. Moreover, the center of a track formed in a spiral manner is not necessarily in the axis of a disc motor, which results from improper placement of the disc when the disc is loaded (clamped). Furthermore, in manufacturing a disc drive, it is probable that the rotor axis of the disc motor for rotating the disc is not in the actual axis of rotation. Moreover, the rotor axis of the disc motor may have inclination to the actual axis of rotation.
The effect of such eccentricity of the disc rotation on the track search will be considered. When the track search is performed in an ideal condition without the eccentricity and if an objective lens housed in the pickup is moved at a constant velocity toward the inward direction/outward direction of the disc with respect to a frame fixing the mechanism, the relative velocity between the track and the objective lens becomes also constant. However, when the track search is performed under a condition with the existence of the eccentricity, if the objective lens is moved at a constant velocity with respect to the frame, the relative velocity between the track and the objective lens is modulated by the eccentric acceleration.
Therefore, in order to keep the relative velocity between the track and the objective lens constant or in a target velocity, it is necessary to alter the acceleration/deceleration energy that is given to a tracking coil (a drive coil of a pickup sending motor) in response to this change of the eccentric acceleration.
In this case, in the velocity control method as described above, the velocity measurement is performed for every half-track period by counting the target velocity clock FVCK and this count data is used as the track search velocity error data (acceleration/deceleration data). However, regarding the velocity measurement, the velocity measurement result for previous half-track period is used and the acceleration/deceleration data is outputted in a half-track period next to the velocity measuring half-track period, and hence this method may arise a problem.
The reason for this is that, since the variation of the eccentric acceleration is large in the multi-speed reproduction, the relative velocity between the track and the lens for the half-track period when the track search velocity is measured may differ largely from that for a half-track period when the acceleration/deceleration data is actually outputted, which results from the modulation due to the eccentricity.
That is, in an example shown in FIG. 15, at a time when the count value of the FVCTR 81 reaches C5, the track search control circuit is going to operate in such a way that almost maximum acceleration energy in the outward direction is made to be outputted, however in the next half-track period when the acceleration/deceleration data for outputting this acceleration energy is actually outputted, the track search velocity has already become approximately close to the target velocity.
The worst case is a case where the track search velocity is too high compared to the target velocity at the time of the detection of the track velocity, and the track search velocity, namely the relative velocity between the track and the objective lens, is controlled so as to become lower because of the erroneous measurement of the track search velocity resulted from the modulation due to the eccentricity in the next half-track period when the acceleration/deceleration data for causing the deceleration energy to output is actually outputted. In this case, although the track search velocity is low, the deceleration energy is to be further applied. However, since the acceleration/deceleration data is not renewed until the light beam traverses the track, there is likely to occur a situation where the deceleration energy term is kept on to be applied. In this case, the acceleration/deceleration control gets into an oscillation state and it may be difficult that the track search is stably performed.